This invention relates to a neutral absorbing film suitable for use as a coating on a glass substrate. More particularly, this invention relates to an energy absorbing and anti-reflective coated glass article.
Coatings on glass are commonly utilized to provide specific energy attenuation and light transmittance properties. Additionally, coatings are designed to reduce reflections from interfaces between individual coating layers and the glass when a plurality of coatings are applied onto a glass substrate. The coated articles are often utilized singularly, or in combination with other coated articles, to form a glazing.
The attributes of a coated glass substrate are dependent upon the specific coatings applied to the glass substrate. The coating compositions and thicknesses impart energy absorption and light transmittance properties within the coated article while also affecting the spectral properties. Desired attributes may be obtainable by adjusting the compositions or thicknesses of the coating layer or layers. However, adjustments to enhance a specific property can adversely impact other transmittance or spectral properties of the coated glass article. Obtaining desired spectral properties is often difficult when trying to combine specific energy absorption and light transmittance properties in a coated glass article.
Anti-reflective coatings on glass are utilized to reduce the surface reflection of optical components and to reduce the reflectance of an interface between optical media with different refractive indices. The reduction of visible reflection is achieved by the principle of optical interference. When light impinges on the air-film, film-film, and film-glass interfaces, a portion of the beam is reflected at each interface. By proper choice of thin film materials and thicknesses, the individual reflected light beams can destructively interfere thereby reducing the observed visual reflectance.
The utilization of a coating having absorption properties enables further reduction in reflection by absorbing the light as it travels through the high index absorbing film thereby reducing the light energy incident on the back glass interface and glass-film interface. The absorption of visible light results in the reduction of visible light transmitted through the glass. Generally, absorbing films are strongly colored and therefore do not result in a neutral transmittance or reflectance. The utilization of an energy absorbing film is preferred when the minimization of visible reflection is desired and a reduction of visible light transmittance is acceptable.
Absorbing films may also adversely impact the visible light transmittance to a level unacceptable for anti-reflective and solar control applications. For example, European Patent publication EP0780346 A1 discloses a method for producing tin oxide films doped with antimony oxide. The films are applied pyrolytically and result in a film having a molar ratio of tin to antimony of 1:0.2 to 1:0.5. The resulting films, when applied onto a neutral glass substrate at a thickness of about 50 nm to about 1,500 nm, result in a visible light transmittance of less than 10 percent. The color of the films are generally a dark, gray-violet color. Thus, the low visible light transmittance and spectral properties renders such films undesirable for anti-reflective glass applications.
International Patent Publication No. WO 9902336A (PCT/US98/13531) describes a non-conductive and energy absorbing coating of an antimony/tin oxide alloy. In WO 9902336A, the energy absorbing film, having a refractive index of about 1.8 to about 2.6, may be utilized with a metal oxide, having a lower refractive index, to form a coated glass article, the high refractive index film is applied closest to the glass with the low refractive index film functioning as an outer layer. The high/low stack reduces visible reflection to a level below five percent by the principle of optical interference. Additionally, the absorbing properties of the film enable a further reduction in visible reflection to a level below two percent. The thicknesses and optical characteristics of the coating stack may be adjusted to achieve a broad range of specified transmittance values. In a preferred embodiment of WO 9902336A, the coated glass article has a visible light transmittance (Ill C) of at least 30%. The reflection and transmittance of visible light are both aesthetically neutral in color.
It has thus been known for anti-reflective coating stacks to have an absorbing layer and a conductive layer overlaying the absorbing layer. A preferred application of such a coating stack has been in a screen of a cathode ray tube, for example a computer monitor. The anti-reflective film minimizes the glare reflected by the screen, and the conductive coating acts to dissipate static electricity.
It would be advantageous to provide a coated glass article having an energy absorbing film that is capable of still further reducing the visible reflection from the glass while permitting a visible light transmittance of at least 30 percent. The film should also provide a desirable neutral color in both transmittance and reflectance.
It would be advantageous to provide low reflected glare with contrast enhancement between the background and screen text of display screen, and further to provide means for dissipating a static charge for the display screen.
It would be a further advantage to provide a color neutral absorbing film that may be applied pyrolytically onto a glass substrate. A pyrolytic film enables the deposition of the film on-line, for example, in a float glass production process.